A lot of ISPs include a performance test built into the modem which the customer and employees can access via a portal. For high speed plans e.g. ATT will have the installer who brings the modem, connects it, runs the test, shows the customer the speed, and then asks the customer to test their device. From there they can troubleshoot/explain any differences. Particularly around the >1 gbps plans because so few people realize most of their wired devices have that limit.
Of course if you're a "light initial touch" ISP or are more open about letting them run their own modem then this kind of approach isn't as easy to do and you're stuck with trying to educate the customer on doing the whole shebang themselves.
Yes, we are doing all of the above except for the on-modem speed test, which we are working on.
Having the installer run a test and talk with the customer is helpful, but a lot of people don't trust the installer. The incumbent ISPs are so evil that they've convinced consumers that all ISPs are evil tricksters. So we'll get a customer service call 10 minutes after the installer has left: "I'm not getting the speeds I'm paying for!"
Of course if you're a "light initial touch" ISP or are more open about letting them run their own modem then this kind of approach isn't as easy to do and you're stuck with trying to educate the customer on doing the whole shebang themselves.